When measuring the arterial blood pressure, the pressure in an arterial blood vessel of a human being or an animal is measured. In medical diagnosis it plays a key role for a large number of illnesses, in particular of the cardiovascular system.
Furthermore, the direct, invasive measurement of blood pressure by means of a pressure sensor having a direct hydraulic connection with the blood in a blood vessel is distinguished from the indirect, non-invasive measurement of blood pressure which is usually carried out at one of the extremities with the aid of a pneumatic cuff.
In the case of direct measurement, a blood vessel, mostly a peripheral artery, is punctured, and a catheter inserted. The latter is connected to a pressure sensor via a tube filled with liquid, through which the arterial blood pressure curve can be displayed on a monitor. The measurement is exact, and offers the benefit of continual monitoring, as long as it is guaranteed that an ongoing column of liquid between the blood vessel and the pressure sensor, not containing any blood clot or air bubble, exists, and the pressure hose has the necessary stiffness, which is not always a given in clinical practice. In addition, the heart rate, the systolic, diastolic and mean arterial blood pressure (SAP, DAP, MAP) and the pulse pressure variation (PPV), as well as—by means of the pulse contour method—the cardiac output (PCCO) and the stroke volume variation (SVV) can be determined. Since the method is time-consuming and invasive, it causes considerably increased costs and is associated with the risk of bleeding, hematomas, thromboembolisms, infections and nerve injuries. It is especially utilized for monitoring purposes during an operation, in intensive care units and in the cardiac catheter laboratory, however usually not outside the scope of these applications.
With indirect arterial pressure measurement, the arterial pressure is measured at one of the extremities, mostly on the arm, with the aid of a blood pressure monitor. Although indirect measurements are not as precise as direct measurements of the blood pressure, the light, rapid, safe and cost-effective implementation makes them the means of choice with most medical applications. The manual or automated measurement may be performed auscultatorily, on palpation and oscillatorily.
In the case of the auscultatory measurement, a pneumatic pressure cuff is inflated on the upper arm up to above the anticipated arterial pressure. When the cuff pressure is slowly lowered, the occurrence, and thereafter the disappearance again of so-called “Korotkoff sounds” can be heard above the artery of the arm with the aid of a stethoscope. The value which can be read off on the scale of the manometer when acoustically perceiving the Korotkoff sounds for the first time corresponds to the upper, systolic blood pressure. The systolic blood pressure is, at this moment, greater than the pressure in the cuff. The pressure in the cuff is further reduced with a suitable speed. Should it fall short of the diastolic arterial pressure, the Korotkoff sounds disappear. This value corresponds to the diastolic blood pressure. The auscultatory measurement still serves as a reference procedure in the case of non-invasive measurement procedures.
Also in the case of measurement on palpation, a pressure cuff is placed on the upper arm. When the cuff pressure is reduced again, the pulse is felt at the radial artery. The value which can be read off on the scale of the measuring device when feeling the pulse for the first time corresponds to the systolic blood pressure. The diastolic blood pressure cannot be determined in this way. The method is in particular used for an application in a loud environment or if no stethoscope is to hand.
With the oscillatory measurement, the artery in the upper arm or wrist or leg is pressed with the aid of a pressure cuff. While the air is slowly being let out of the pressure cuff, the blood begins to flow through the artery again. In the process, vibrations in the arterial walls, which are triggered by the flow of blood beginning, can be recorded. Such vibrations, also known as oscillations, first become stronger, then weaker, and finally fade out altogether once the blood flows through the blood vessels again. The vibrations are transmitted to the pressure cuff, and in this way lead to an oscillating needle deflection of the manometer. The maximum and minimum values of the needle deflection chronologically correspond to the systolic and diastolic blood pressure. From the cuff pressure at the point in time of the maximum oscillations and the full width at half maximum, the systolic and diastolic pressure can be calculated. When performing a manual measurement, only imprecise results are achieved with this method. This method is, however, utilized in automatic blood pressure measurement devices, in particular also with permanent monitoring, e.g. intraoperatively and/or postoperatively, in the recovery room. This involves that, as an alternative to the continuous invasive pressure measurement, the blood pressure measurement devices measure the arterial blood pressure of the patient intermittently, at intervals of a few minutes. When measuring the vibrations determined electronically, the systolic and diastolic blood pressure are calculated mathematically, with the help of an algorithm, from the curve progression of the vibrations.
According to EP 0 467 853 B1, it is disclosed in the publication, “Possible determinants of pulse wave velocity in vivo”, Masahiko Okada, IEEE Transactions on Biomedical Engineering, Vol. 35, No. 5, May 1988, pp. 357 to 361, that a certain correlation between the pulse wave velocity and the systolic and diastolic blood pressure can be noticed. Such a weak, low correlation is, however, according to this teaching, said to not make it possible to determine the blood pressure.
On this basis, a method of continuously determining the blood pressure of a patient is proposed in EP 0 467 853 B1, in the case of which, using an electronic measuring device, which contains two sensors, arranged at a defined distance, at least one factor which changes chronologically with the beat of the pulse, is determined, which is a measure of the flow velocity, the flow rate, the volume of the arterial blood or the cross-sectional area of outlet orifice flow of an arterial blood vessel, and a second factor is determined, which is a measure of the pulse wave velocity of a pressure wave in the blood in said blood vessel caused by a heartbeat. The blood pressure is obtained by means of both factors, taking calibration values into account. In that respect, it could, for example, be made use of the fact that the blood pressure is proportional to the flow velocity of the blood. The quotient “kv/r2” serves, in that regard, as a proportionality factor, wherein “kv i” s a constant, and “r” the radius of the vessel section observed (assuming a circular cross section). Were “r” a constant, the blood pressure could be determined directly from multiple measurements of the flow velocity. As the vessel wall is, however, elastic, and “r” is accordingly not constant, but highly variable, it is necessary to include the measurement of the pulse wave velocity into the formula. In that respect, use is made of the fact that the module of elasticity of the vessel wall can be determined from the pulse wave velocity.
The functionality of the sensors in the case of the method disclosed in EP 0 467 853 B1 may be optically based. To this end, they may each have an optoelectronic transformer, serving as a spotlight, e.g. a laser light-emitting diode, and an optoelectronic transformer serving as a light receptor, such as a photodiode. Both sensors may be integrated into a cuff and attached to one of a patient's forearms, for example, in order to ascertain the blood pressure. A calibration of the sensor-based determination of the blood pressure can, in line with a conventional, non-invasive method according to Riva-Rocci, be performed with an inflatable cuff. The latter involves the cuff being placed at a different extremity of the patient than is stipulated for the sensor cuff, to avoid the sensor-based measurements being falsified. Calibration can be undertaken once at the beginning of the sensor-based long-duration measurement, or repeatedly, after a respective defined period of time, such as daily.
US 2013/0079648 A1 discloses a method and a device for measuring the pulse wave velocity in a vein, wherein a bulging out of the vein below the sensors induced by pressure is detected underneath the sensors using two pressure sensors that are integrated into a cuff, for example, at a defined short distance. The pulse wave transit time can be determined by means of sensors, if, for instance, the time difference of the ascertaining of a pressure maximum allocated to the same pulse wave is drawn upon by the two sensors. From the pulse wave transit time, a conclusion can then be drawn about the pulse wave transit time in connection with the known distance between the sensors.
In US 2013/0079648 A1 it is, moreover, disclosed that a conclusion can also be drawn about the blood pressure based on the (two) pressure sensors. This involves that the respective voltage of the two pressure sensors that is measured is converted into a value for the blood pressure based on a transformation coefficient matrix. The average value from the two values of the two pressure sensors is then taken to be the value determined for the blood pressure. Since the transformation coefficient matrix is generally not known, and can change over time, this method is practically not implementable.